[Merge] ~gjolly/livecd-rootfs:ubuntu-cpc/device_busy into livecd-rootfs:ubuntu/master

[dann frazier]

wait_on_device_busy() seems inherently racy, because there's nothing guaranteeing that our ghost user won't sneak in between wait_on_device_busy() returning and us trying e2fsck again.

I wonder if this is a similar issue to 2045586, where we are racing with systemd-udevd.  The systemd docs do say we should be flocking the full block device around operations like fsck on partitions:

  https://systemd.io/BLOCK_DEVICE_LOCKING/

You could try to use ftrace to prove who the other user is, like I did here:
  https://launchpad.net/bugs/2045586/comments/11
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https://code.launchpad.net/~gjolly/livecd-rootfs/+git/livecd-rootfs/+merge/448708
Your team Ubuntu Core Development Team is requested to review the proposed merge of ~gjolly/livecd-rootfs:ubuntu-cpc/device_busy into livecd-rootfs:ubuntu/master.